Our aim is to understand the structure-function relationship of modified nucleosides and nucleotides in nucleic acids. Our objective is to establish by x-ray diffraction techniques the three dimensional structures of mRNA caps, oligo 2-5A's that are the signal of dsRNA in interferon treated cells and of short pieces of oligonucleotides with and without mutagens and carcinogens intercalated or covalently bound. The stereochemistry of 2'-5' and 5'-5' phosphodiester links will be studied and will be compared with the stereochemistry of 3'-5' links. Our aim is to understand why 3'-5' phosphodiester bonds and not the 2'-5' phosphodiester bonds are used in all genetic information transfer though the 2'-5' links are formed more readily. Our studies on the oligonucleotides modified with carcinogens will throw light on the basic mechanism of carcinogenesis.